It is known to prepare polyperoxy perfluoropolyethers by photochemical oxidation of fluorolefins as it is described in particular in British Pat. No. 1,104,482.
It is also known to prepare perfluoropolyethers stable at high temperatures, obtained from the above polyperoxy perfluoropolyethers by heating them or by using suitable reducing compounds as it is described in particular in British Pat. No. 1,226,566.
By using perfluoropropene as starting compound, in the processes described in the two above patents, compounds of general formula: ##STR1## are obtained, wherein A is --CF.sub.3, --C.sub.2 F.sub.5, --C.sub.3 F.sub.7 and Z is an acid group ##STR2## or a derivative thereof or a group --CO--CF.sub.3.
The monomeric units with indexes m, n and r are randomly distributed along the chain of the perfluoropolyether; m, n and r are integers from 0 to 200, their values depending on the photo-oxidation reaction conditions, and the value of m+n+r being always greater than zero.
By using tetrafluoroethylene as starting compound perfluoropolyethers of general formula: EQU B--O(--CF.sub.2 CF.sub.2 --O).sub.p --(CF.sub.2).sub.q --Z (II)
are obtained, wherein B is --CF.sub.3, --C.sub.2 F.sub.5 and Z is an acid group ##STR3## or a derivative thereof; the monomeric units with indexes p and q are randomly distributed along the chain of the perfluoropolyether; p and q are integers and p/q is comprised between 0.5 and 2 (extremes included), and their values depend on the photo-oxidation reaction conditions.
There are also known perfluoropolyethers obtained by polymerization in the presence of anionic catalysts of perfluoropropene epoxide, as it is described for example in U.S. Pat. No. 3,250,808, having general formula: ##STR4## wherein Y is an acyl fluoride group ##STR5## and m is an integer greater than zero.
The perfluoropolyethers of the three classes above can be subjected to known treatments in order to obtain neutral perfluoropolyethers. An example of the above treatments consists in heating the perfluoropolyether at temperatures of from 100.degree. to 200.degree. C. in the presence of fluorine.
The general formulae of the perfluoropolyethers obtained are: ##STR6## in which A and A', equal or different, can be --CF.sub.3, --C.sub.2 F.sub.5 or --C.sub.3 F.sub.7 ; m, n, and r have the meaning indicated above; EQU B--O--(CF.sub.2 CF.sub.2 O).sub.p --(CF.sub.2 O).sub.q --B'(V)
in which B and B', equal or different, are --CF.sub.3 or --C.sub.2 F.sub.5 ; p and q have the meaning already indicated; EQU C.sub.3 F.sub.7 --O--(C.sub.3 F.sub.6 O).sub.m --R.sub.f (VI)
in which R.sub.f is --C.sub.2 F.sub.5 or --C.sub.3 F.sub.7 ; m is an integer greater than zero.